High temperature strength, abrasion resistance and corrosion/oxidation resistance are required for materials of exhaust valves, which are generally subjected to temperatures exceeding 800° C. The exhaust valves used in most reciprocating engines can generally be divided into three sections; the head, stem and stem tip. The head and a portion of the head leading from the stem consist of a high temperature, high strength and corrosion resistant alloy such as an austenitic stainless steel or a superalloy. The sealing surface of the valve often includes a weld overlay material, such as a cobalt based, high temperature alloy. The remainder of the stem often is made of a hardenable martensitic steel welded to the high-temperature heat-resistant alloy of the valve head end.
As improved internal combustion engines are developed, addressing the increasing temperatures resulting from higher fuel economy, reduced emissions and yet higher output through newly designed engines has prompted the need for new cost effective materials. In addition, because the demand for and cost of nickel is on the rise, alternatives for high nickel content alloys are desired.
Austenitic stainless steels such as 21-2N, 21-4N-Nb-W and 23-8N have been used for the manufacture of engine valves for many decades. However, due to mechanical property limitations, these alloys are not suitable at operating temperatures above 1472° F. (800° C.) for current durability expectations.
Superalloys, including Fe—Ni-based and Ni-based alloys, have been used for exhaust valve applications typically when the less expensive iron-based stainless valve steel would not provide sufficient high-temperature strength or corrosion resistance, or both, for a given application. Some of the higher nickel alloys used for valve applications include Alloy 751, Alloy 80A, Pyromet 31 and Ni30, for example. Alloys 751, 80A and Pyromet 31 contain high amounts of Ni and are therefore expensive. Valves manufactured from these higher content Ni alloys are susceptible to abrasive and adhesive wear on the seat face due to the lack of wear resistance. Therefore, valves manufactured from some of the higher Ni alloys must be hard faced with a Co-based alloy on the seat face to improve wear resistance. This adds a manufacturing step that further increases the cost of the valve. Thus, there is a need for an intermediate strength valve alloy with properties and cost between that of the austenitic valve steels and the Ni-based superalloys such that the alloy has sufficient wear resistance without requiring a hard facing step.